Author Richard Seymour, who has written extensively on changes inside the Labour Party, believes those abandoning ship could be a mix of “centrists” who dislike Corbyn, as well as “disappointed Remainers” and “disaffected Corbynites.”

“Without any polling on that, my instinct says mostly the first two,” he wrote on Twitter.

Leaked figures show party membership peaked at 554,000 last July.

By January 2017, a report from the party’s general secretary, Iain McNicol, had the membership at 543,345.

“This is the highest figure on Labour Party records,” National Executive Committee member Peter Willsman said at the time.

Alex Nunns, author of The Candidate, Jeremy Corbyn’s Improbable Path to Power, told RT: “While thousands may have left over Brexit, infighting and Corbyn’s reelection – others will have died, got ill, become skint, or moved away.

“Up to that point they were outnumbered by new people continuing to join. So the 26,000 figure is rather meaningless.”

“I imagine people are losing heart because they can see the polls, they’re talking to their neighbors and people they work with, and are coming to the conclusion Labour is not doing well and, at this point, not convincing the public.”

Labour controlled the Copeland seat for more than 80 years before the disastrous defeat.

In Kersal, Salford, the Labour vote slumped by 21.5 points. Given the area’s strong Jewish community, the LabourList blog suggested the party lost “credibility in the community as a result of the poor handling of anti-Semitism complaints.”